Renal effects from limitation of high dietary protein in normoalbuminuric diabetic patients.

Glomerular hyperfiltration may be a risk factor for late nephropathy. It has been shown that considerable protein restriction can lower glomerular filtration rate (GFR). To elucidate the effect of moderate protein limitation in type 1 (insulin-dependent) diabetics with normoalbuminuria, eight such patients were selected for the study (age 38 +/- 7 years SD, diabetes duration 21 +/- 9 years). The patients were randomized to follow, alternately, four weeks of their usual protein intake (19% of energy) and four weeks of a limited protein intake (12% of energy). Kidney function was investigated after the two dietary periods. GFR and renal plasma flow (RPF) were measured using a constant infusion technique (125I-iothalamate/131I-hippuran), and urinary albumin excretion (UAE) by radioimmunoassay. It was found that the limited protein diet reduced GFR from 146 +/- 23 to 132 +/- 24 ml/min/1.73 m2 (2P less than 0.005). A tendency towards a fall in RPF was seen (549 +/- 128 vs. 503 +/- 125 ml/min; 2P = 0.06), while total renal resistance rose from 0.17 +/- 0.03 to 0.20 +/- 0.05 mm Hg/ml/min (2P = 0.05). No significant changes in filtration fraction, UAE and blood pressure were seen. HbA1c, fructosamine, insulin dose and body weight were unchanged during the two diets; also serum protein, albumin, phosphate and calcium remained unaltered. Serum urea was significantly reduced on the limited protein intake. Patients were generally pleased with the limited protein diet. Thus, limitation of the often high protein intake in diabetics might be valuable and realistic. The long-term renal protective effect remains to be investigated.